Silicon carbide epitaxial substrate and method of manufacturing silicon carbide semiconductor device

ABSTRACT

A silicon carbide epitaxial substrate has a silicon carbide single-crystal substrate and a silicon carbide layer. An average value of carrier concentration in the silicon carbide layer is not less than 1×10 15  cm −3  and not more than 5×10 16  cm −3 . In-plane uniformity of the carrier concentration is not more than 2%. The second main surface has: a groove  80  extending in one direction along the second main surface, a width of the groove in the one direction being twice or more as large as a width thereof in a direction perpendicular to the one direction, and a maximum depth of the groove from the second main surface being not more than 10 nm; and a carrot defect. A value obtained by dividing a number of the carrot defects by a number of the grooves is not more than 1/500.

TECHNICAL FIELD

The present disclosure relates to a silicon carbide epitaxial substrateand a method of manufacturing a silicon carbide semiconductor device.The present application claims priority to Japanese Patent ApplicationNo. 2015-228601 filed on Nov. 24, 2015, the entire contents of which areincorporated herein by reference.

BACKGROUND ART

Japanese Patent Laying-Open No. 2014-170891 (PTD 1) discloses a methodof epitaxially growing a silicon carbide layer on a silicon carbidesingle-crystal substrate.

CITATION LIST Patent Document

PTD 1: Japanese Patent Laying-Open No. 2014-170891

SUMMARY OF INVENTION

A silicon carbide epitaxial substrate according to the presentdisclosure includes a silicon carbide single-crystal substrate and asilicon carbide layer. The silicon carbide single-crystal substrateincludes a first main surface. The silicon carbide layer is on the firstmain surface. The silicon carbide layer includes a second main surfaceopposite to a surface thereof in contact with the silicon carbidesingle-crystal substrate. An average value of carrier concentration inthe silicon carbide layer is not less than 1×10¹⁵ cm⁻³ and not more than5×10¹⁶ cm⁻³. In-plane uniformity of the carrier concentration is notmore than 2%. The second main surface has: a groove extending in onedirection along the second main surface, a width of the groove in theone direction being twice or more as large as a width thereof in adirection perpendicular to the one direction, and a maximum depth of thegroove from the second main surface being not more than 10 nm; and acarrot defect. A value obtained by dividing a number of the carrotdefects by a number of the grooves is not more than 1/500.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic plan view showing the configuration of a siliconcarbide epitaxial substrate according to the present embodiment.

FIG. 2 is a schematic sectional view showing the configuration of thesilicon carbide epitaxial substrate according to the present embodiment.

FIG. 3 is a schematic plan view showing measurement positions of carrierconcentration according to the present embodiment.

FIG. 4 is a schematic plan view showing the configuration of a minutepit according to the present embodiment.

FIG. 5 is a schematic sectional view showing the configuration of theminute pit according to the present embodiment.

FIG. 6 is a schematic plan view showing the configuration of a carrotdefect according to the present embodiment.

FIG. 7 is a schematic sectional view showing the configuration of thecarrot defect according to the present embodiment.

FIG. 8 is a partial schematic sectional view showing the configurationof an apparatus of manufacturing the silicon carbide epitaxial substrateaccording to the present embodiment.

FIG. 9 is a flowchart schematically showing a method of manufacturing asilicon carbide semiconductor device according to the presentembodiment.

FIG. 10 is a schematic sectional view showing a first step of the methodof manufacturing the silicon carbide semiconductor device according tothe present embodiment.

FIG. 11 is a schematic sectional view showing a second step of themethod of manufacturing the silicon carbide semiconductor deviceaccording to the present embodiment.

FIG. 12 is a schematic sectional view showing a third step of the methodof manufacturing the silicon carbide semiconductor device according tothe present embodiment.

FIG. 13 is a schematic sectional view showing a first step of a methodof manufacturing a silicon carbide single-crystal substrate according tothe present embodiment.

FIG. 14 is a schematic sectional view showing a second step of themethod of manufacturing the silicon carbide single-crystal substrateaccording to the present embodiment.

FIG. 15 is a diagram showing relation between a minute pit density in asecond main surface of the silicon carbide single-crystal substrate anda wafer number of silicon carbide single-crystal substrates.

FIG. 16 is a diagram showing relation between a carrot defect density inthe second main surface of the silicon carbide single-crystal substrateand a wafer number of silicon carbide single-crystal substrates.

DESCRIPTION OF EMBODIMENTS Overview of Embodiment of the PresentDisclosure

An overview of an embodiment of the present disclosure is describedfirst. In the following description, the same or corresponding elementsare designated by the same symbols and the same description thereof willnot be repeated. Regarding crystallographic indications herein, anindividual orientation is represented by [ ], a group orientation isrepresented by < >, an individual plane is represented by ( ) and agroup plane is represented by { }. Although a crystallographicallynegative index is normally expressed by a number with a bar “-”thereabove, a negative sign herein precedes a number to indicate acrystallographically negative index.

(1) A silicon carbide epitaxial substrate 100 according to the presentdisclosure includes a silicon carbide single-crystal substrate 10 and asilicon carbide layer 20. Silicon carbide single-crystal substrate 10includes a first main surface 11. Silicon carbide layer 20 is on firstmain surface 11. Silicon carbide layer 20 includes a second main surface12 opposite to a surface 14 thereof in contact with silicon carbidesingle-crystal substrate 10. An average value of carrier concentrationin silicon carbide layer 20 is not less than 1×10¹⁵ cm⁻³ and not morethan 5×10¹⁶ cm⁻³. In-plane uniformity of the carrier concentration isnot more than 2%. Second main surface 12 has: a groove 80 extending inone direction along second main surface 12, a width of the groove in theone direction being twice or more as large as a width thereof in adirection perpendicular to the one direction, and a maximum depth of thegroove from the second main surface being not more than 10 nm; and acarrot defect 90. A value obtained by dividing a number of carrotdefects 90 by a number of grooves 80 is not more than 1/500.

Usually, a plurality of threading screw dislocations are present in asilicon carbide single-crystal substrate. When forming a silicon carbidelayer on a silicon carbide single-crystal substrate by epitaxial growth,threading screw dislocations present in the silicon carbidesingle-crystal substrate are transferred to the silicon carbide layer.Controlling the conditions for the epitaxial growth allows most of thethreading screw dislocations transferred to the silicon carbide layer toappear as minute pits at a surface of the silicon carbide layer. On theother hand, some of the threading screw dislocations transferred to thesilicon carbide layer change the direction in which they extend due tothermal fluctuations and the like during the epitaxial growth, forexample, and appear as carrot defects at the surface of the siliconcarbide layer. The carrot defects cause deterioration in breakdownvoltage of a silicon carbide semiconductor device, whereas the minutepits have little effect on breakdown voltage.

As a result of research, the inventors found that a method describedlater allows most of threading screw dislocations to appear as minutepits at a surface of a silicon carbide layer, and can suppress theappearance of the threading screw dislocations as carrot defects at thesurface. Accordingly, a silicon carbide epitaxial substrate in which avalue obtained by dividing the number of carrot defects by the number ofgrooves (also referred to as minute pits hereinafter) is not more than1/500 can be provided. As a result, deterioration in breakdown voltageof a silicon carbide semiconductor device manufactured with this siliconcarbide epitaxial substrate can be suppressed.

(2) In silicon carbide epitaxial substrate 100 according to (1) above,groove 80 may include a first groove portion 81 and a second grooveportion 82 provided continuously with first groove portion 81. Firstgroove portion 81 is at one end portion of the groove in the onedirection. Second groove portion 82 may extend from first groove portion81 along the one direction to the other end portion opposite to the oneend portion, and have a depth from second main surface 12 which issmaller than a maximum depth of first groove portion 81.

(3) In silicon carbide epitaxial substrate 100 according to (1) or (2)above, the value obtained by dividing the number of carrot defects 90 bythe number of grooves 80 may be not more than 1/1000.

(4) In silicon carbide epitaxial substrate 100 according to (3) above,the value obtained by dividing the number of carrot defects 90 by thenumber of grooves 80 may be not more than 1/5000.

(5) In silicon carbide epitaxial substrate 100 according to any one of(1) to (4) above, a density of carrot defects 90 in second main surface12 may be not more than 1 cm⁻².

(6) In silicon carbide epitaxial substrate 100 according to (5) above,the density of carrot defects 90 may be not more than 0.5 cm⁻².

(7) In silicon carbide epitaxial substrate 100 according to (6) above,the density of carrot defects 90 may be not more than 0.1 cm⁻².

(8) A method of manufacturing a silicon carbide semiconductor device 300according to the present disclosure includes the following steps.Silicon carbide epitaxial substrate 100 according to any one of (1) to(7) above is prepared. Silicon carbide epitaxial substrate 100 isprocessed.

DETAILS OF EMBODIMENT OF THE PRESENT DISCLOSURE

The details of the embodiment of the present disclosure are nowdescribed.

(Silicon Carbide Epitaxial Substrate)

As shown in FIGS. 1 and 2, a silicon carbide epitaxial substrate 100according to the present embodiment has a silicon carbide single-crystalsubstrate 10 and a silicon carbide layer 20. Silicon carbidesingle-crystal substrate 10 includes a first main surface 11, and athird main surface 13 opposite to first main surface 11. Silicon carbidelayer 20 includes a fourth main surface 14 in contact with siliconcarbide single-crystal substrate 10, and a second main surface 12opposite to fourth main surface 14. As shown in FIG. 1, silicon carbideepitaxial substrate 100 may have a first flat 1 extending in a firstdirection 101. Silicon carbide epitaxial substrate 100 may have a secondflat (not shown) extending in a second direction 102. First direction101 is a <11-20> direction, for example. Second direction 102 is a<1-100> direction, for example.

Silicon carbide single-crystal substrate 10 (which may be abbreviated as“single-crystal substrate” hereinafter) is made of a silicon carbidesingle crystal. This silicon carbide single crystal has a polytype of4H—SiC, for example. The 4H—SiC has better electron mobility, dielectricbreakdown electric field strength and the like than other polytypes.Silicon carbide single-crystal substrate 10 includes an n type impuritysuch as nitrogen. Silicon carbide single-crystal substrate 10 has n typeconductivity, for example. First main surface 11 is a {0001} plane or aplane tilted not more than 8° from the {0001} plane, for example. Whenfirst main surface 11 is tilted from the {0001} plane, a tilt directionof the normal of first main surface 11 is the <11-20> direction, forexample.

Silicon carbide layer 20 is an epitaxial layer formed on silicon carbidesingle-crystal substrate 10. Silicon carbide layer 20 is on first mainsurface 11. Silicon carbide layer 20 is in contact with first mainsurface 11. Silicon carbide layer 20 includes an n type impurity such asnitrogen (N). Silicon carbide layer 20 has n type conductivity, forexample. The concentration of the n type impurity included in siliconcarbide layer 20 may be lower than the concentration of the n typeimpurity included in silicon carbide single-crystal substrate 10. Asshown in FIG. 1, second main surface 12 has a maximum diameter 111(diameter) of not less than 100 mm, for example. The diameter of maximumdiameter 111 may be not less than 150 mm, not less than 200 mm, or notless than 250 mm. The upper limit of maximum diameter 111 is notparticularly limited. The upper limit of maximum diameter 111 may be 300mm, for example.

Second main surface 12 may be a {0001} plane or a plane tilted not morethan 8° from the {0001} plane, for example. Specifically, second mainsurface 12 may be a (0001) plane or a plane tilted not more than 8° fromthe (0001) plane. Alternatively, second main surface 12 may be a (000-1)plane or a plane tilted not more than 8° from the (000-1) plane. A tiltdirection (off direction) of the normal of second main surface 12 may bea <11-20> direction, for example. A tilt angle (off angle) from the{0001} plane may be not less than 1°, or not less than 2°. The off anglemay be not more than 7°, or not more than 6°.

(In-Plane Uniformity of Carrier Concentration)

Silicon carbide layer 20 includes nitrogen, for example, as a dopant. Anaverage value of carrier concentration in silicon carbide layer 20 isnot less than 1×10¹⁵ cm⁻³ and not more than 5×10¹⁶ cm⁻³. The averagevalue of the carrier concentration may be not more than 2×10¹⁶ cm⁻³, notmore than 1×10¹⁶ cm⁻³, not more than 9×10¹⁵ cm⁻³, or not more than8×10¹⁵ cm⁻³. The average value of the carrier concentration may be notless than 5×10¹⁵ cm⁻³, or not less than 6×10¹⁵ cm⁻³, for example.

The carrier concentration as used herein means an effective carrierconcentration. If the silicon carbide layer includes a donor and anacceptor, for example, the effective carrier concentration is calculatedas an absolute value of the difference between a donor concentration(N_(d)) and an acceptor concentration (N_(a)) (|N_(d)−N_(a)|).

In-plane uniformity of the carrier concentration in silicon carbidelayer 20 is not more than 2%. The in-plane uniformity of the carrierconcentration is a ratio (σ/ave) of a standard deviation of the carrierconcentration to an average value of the carrier concentration insilicon carbide layer 20 in a direction parallel to second main surface12. The in-plane uniformity of the carrier concentration may be not morethan 1.5%, or not more than 1%.

Next, a method of measuring the carrier concentration is described. Thecarrier concentration is measured by a mercury probe type C-Vmeasurement device, for example. The probe has an area of 0.01 cm², forexample. Second main surface 12 includes an outer edge 3, an outercircumferential region 4, and a central region 5. Outer edge 3 haslinear first flat 1 and a curvature portion 2. Outer circumferentialregion 4 is a region within 5 mm from outer edge 3. Central region 5 isa region surrounded by outer circumferential region 4. The carrierconcentration is measured in central region 5. In other words, thecarrier concentration in outer circumferential region 4 is not measured.Positions obtained by dividing a segment 7 which passes through thecenter of second main surface 12 and is parallel to first direction 101into 12 substantially equal parts in central region 5, for example,serve as measurement positions. Similarly, positions obtained bydividing a segment 6 which passes through the center of second mainsurface 12 and is parallel to second direction 102 into 12 substantiallyequal parts serve as measurement positions. A point of intersection ofthese two straight lines is one of the measurement positions. As shownin FIG. 3, the carrier concentration is measured at the total of 25measurement positions (hatched regions) in central region 5. An averagevalue and a standard deviation of the carrier concentrations at thetotal of 25 measurement positions are calculated.

As shown in FIG. 2, silicon carbide layer 20 includes a surface layerregion 29 and a bottom layer region 26. Surface layer region 29 is aregion within 10 μm from second main surface 12 toward fourth mainsurface 14 in a direction perpendicular to second main surface 12. Adepth of measurement is adjusted by an applied voltage. Bottom layerregion 26 is a region sandwiched between surface layer region 29 andsilicon carbide single-crystal substrate 10. The carrier concentrationis measured in surface layer region 29. The measured data is plottedwith 1/C² as the vertical axis and V as the horizontal axis. The carrierconcentration is estimated from the slope of a line of the measureddata.

(Minute Pit)

Next, the configuration of a minute pit (groove) is described. As shownin FIGS. 4 and 5, second main surface 12 has a groove 80. In plan view(a view seen in the direction perpendicular to second main surface 12),groove 80 extends in one direction along the direction parallel tosecond main surface 12. Specifically, groove 80 extends along astep-flow growth direction 8, which is along an off direction of an offangle relative to a (0001) plane. For example, groove 80 extends along adirection in the range of not more than ±5° relative to a <11-20>direction, or a direction in the range of not more than ±5° relative toa <01-10> direction.

As shown in FIG. 4, a width 117 in the aforementioned one direction ofgroove 80 is twice or more as large as, and preferably, five times ormore as large as, a width 119 thereof in a direction perpendicular tothe aforementioned one direction. Width 117 is not less than 15 μm andnot more than 50 μm, and preferably not less than 25 μm and not morethan 35 μm. Width 119 is not less than 1 μm and not more than 5 μm, andpreferably not less than 2 μm and not more than 3 μm.

As shown in FIG. 5, groove 80 extends from a threading dislocation 25(more specifically, a threading screw dislocation) present in siliconcarbide layer 20, along step-flow growth direction 8 which is along theoff direction of the off angle. More specifically, groove 80 includes afirst groove portion 81 formed on threading dislocation 25, and a secondgroove portion 82 provided continuously with first groove portion 81 andextending from first groove portion 81 along step-flow growth direction8.

First groove portion 81 is formed at one end portion (left end portionin FIG. 5) of groove 80 in step-flow growth direction 8. A maximum depth114 of first groove portion 81 from second main surface 12 is not morethan 10 nm. Maximum depth 114 is the maximum depth in the entire groove80. A width 116 of first groove portion 81 is preferably not more than 1μm, and more preferably not more than 0.5 μm.

As shown in FIG. 5, second groove portion 82 extends from its portion ofconnection with first groove portion 81 to the other end portion (rightend portion in FIG. 5) opposite to the aforementioned one end portion.In other words, second groove portion 82 extends from first grooveportion 81 along one direction 8 to the other end portion opposite tothe one end portion. A depth 113 of second groove portion 82 from secondmain surface 12 is smaller than maximum depth 114 of first grooveportion 81. More specifically, second groove portion 82 extends alongstep-flow growth direction 8 while maintaining depth 113 shallower thanmaximum depth 114 of first groove portion 81. Depth 113 is preferablynot more than 3 nm, more preferably not more than 2 nm, and furtherpreferably not more than 1 nm. A width 118 of second groove portion 82is not less than 20 μm, for example, and preferably not less than 25 μm.

As shown in FIG. 5, a drift layer 20 may include a first layer 23 and asecond layer 24. In a step of forming drift layer 20, epitaxial growthmay be performed in a step forming first layer 23 under conditionsdifferent from those in a step of forming second layer 24. Specifically,a gas flow rate, a film thickness, temperature and the like may bedifferent between the step forming first layer 23 and the step formingsecond layer 24.

(Carrot Defect)

Next, the configuration of a carrot defect is described. As shown inFIG. 6, second main surface 12 has a carrot defect 90. Carrot defect 90is a type of extended defects. An extended defect is a defect having atwo-dimensional extent when viewed from a direction perpendicular to thesurface of a silicon carbide layer. An extended defect may be anextended dislocation formed of two partial dislocations arising from aperfect dislocation, and a stacking fault in the form of a stripconnecting these two partial dislocations, for example.

Carrot defect 90 has an elongate shape when viewed from the directionperpendicular to second main surface 12. A width 115 in a longitudinaldirection of carrot defect 90 depends on the thickness of siliconcarbide layer 20. Typically, width 115 is not less than 100 μm and notmore than 500 μm. As shown in FIG. 6, a width 120 in a transversedirection of carrot defect 90 may decrease monotonously in thelongitudinal direction of carrot defect 90. A maximum value of width 120in the transverse direction is not less than 10 μm and not more than 100μm, for example. Carrot defect 90 has a portion protruding from secondmain surface 12. The protruding portion has a height of not less than0.1 μm and not more than 2 μm, for example.

A value obtained by dividing the number of carrot defects 90 by thenumber of grooves 80 (minute pits 80) is not more than 1/500. The valueobtained by dividing the number of carrot defects 90 by the number ofgrooves 80 is preferably not more than 1/1000, and more preferably notmore than 1/5000.

The density of carrot defects 90 in second main surface 12 is a valueobtained by dividing the number of all carrot defects 90 in second mainsurface 12 by the area (cm²) of second main surface 12. The density ofcarrot defects 90 may be not more than 1 cm⁻², for example. The densityof carrot defects 90 is preferably not more than 0.5 cm⁻², and morepreferably not more than 0.1 cm⁻².

As shown in FIG. 7, threading screw dislocation 25 present in siliconcarbide single-crystal substrate 10 is bent at an interface betweensilicon carbide single-crystal substrate 10 and silicon carbide layer 20(epitaxial layer), and thereby grows to be carrot defect 90. Thus,carrot defect 90 is not generated if threading screw dislocation 25 isstretched without being bent. As shown in FIG. 5, threading screwdislocation 25 stretched to second main surface 12 without being bent atthe interface between silicon carbide single-crystal substrate 10 andsilicon carbide layer 20 causes minute pit 80.

(Method of Measuring the Numbers of Minute Pits and Carrot Defects)

The numbers of “minute pits (grooves)” and “carrot defects” can bemeasured by observing second main surface 12 with a defect inspectiondevice including a confocal differential interference microscope. As thedefect inspection device including the confocal differentialinterference microscope, WASAVI series “SICA 6X” manufactured byLasertec Corporation or the like can be used. The magnification of anobjective lens is set at ×10. A threshold value of detection sensitivityof this defect inspection device is decided using a standard sample.Thus, this defect inspection device can be used to quantitativelyevaluate the numbers of “minute pits” and “carrot defects” that haveappeared at second main surface 12.

For example, central region 5 of second main surface 12 is divided intoa plurality of observed areas. One observed area is a 1.3 mm×1.3 mmsquare region. Images of all observed areas are taken. The image of eachobserved area is processed with a prescribed method, to identify theminute pits and the carrot defects in the image. The number of minutepits and the number of carrot defects are calculated in each of theobserved areas in central region 5, to determine the number of minutepits and the number of carrot defects in the entire central region 5.That the value obtained by dividing the number of carrot defects 90 bythe number of grooves 80 (minute pits 80) is not more than 1/500 meansthat the value obtained by dividing the number of carrot defects by thenumber of minute pits is not more than 1/500 in all observed areas incentral region 5.

(Apparatus of Manufacturing Silicon Carbide Epitaxial Substrate)

Next, the configuration of an apparatus 200 of manufacturing siliconcarbide epitaxial substrate 100 according to the present embodiment isdescribed.

As shown in FIG. 8, manufacturing apparatus 200 is a hot wall typelateral CVD (Chemical Vapor Deposition) apparatus, for example.Manufacturing apparatus 200 mainly has a reaction chamber 201, a gassupply unit 235, a control unit 245, a heating element 203, a quartztube 204, a heat insulator 205, and an induction heating coil 206.

Heating element 203 has a cylindrical shape, for example, and formsreaction chamber 201 therein. Heating element 203 is made of graphite,for example. Heat insulator 205 surrounds the outer circumference ofheating element 203. Heat insulator 205 is provided in quartz tube 204so as to make contact with an inner circumferential surface of quartztube 204. Induction heating coil 206 is wound along an outercircumference surface of quartz tube 204, for example. Induction heatingcoil 206 is configured to be able to supply an alternating current froman external power supply (not shown). Heating element 203 is therebyinductively heated. As a result, reaction chamber 201 is heated byheating element 203.

Reaction chamber 201 is a space formed by being surrounded by heatingelement 203. Silicon carbide single-crystal substrate 10 is disposed inreaction chamber 201. Reaction chamber 201 is configured to be able toheat silicon carbide single-crystal substrate 10. Reaction chamber 201is provided with a susceptor plate 210 holding silicon carbidesingle-crystal substrate 10. Susceptor plate 210 is configured to beable to rotate around its rotation axis 212.

Manufacturing apparatus 200 further has a gas inlet port 207 and a gasoutlet port 208. Gas outlet port 208 is connected to an air exhaust pump(not shown). Arrows in FIG. 8 indicate a gas flow. Gas is introducedthrough gas inlet port 207 into reaction chamber 201, and exhaustedthrough gas outlet port 208. A pressure in reaction chamber 201 isadjusted by a balance between an amount of supplied gas and an amount ofexhausted gas.

Manufacturing apparatus 200 may further have a heating unit 211 locatedbetween gas inlet port 207 and heating element 203. Heating unit 211 islocated upstream of heating element 203. Heating element 203 may beconfigured to be heated to about 1600 K, for example.

Gas supply unit 235 is configured to be able to supply a mixed gasincluding silane, ammonia, hydrogen, and propane, for example, toreaction chamber 201. Specifically, gas supply unit 235 may include afirst gas supply unit 231, a second gas supply unit 232, a third gassupply unit 233, and a carrier gas supply unit 234.

First gas supply unit 231 is configured to be able to supply a first gasincluding carbon atoms. First gas supply unit 231 is a gas cylinderfilled with the first gas, for example. The first gas is propane (C₃H₈)gas, for example. Second gas supply unit 232 is configured to be able tosupply a second gas including silane gas. Second gas supply unit 232 isa gas cylinder filled with the second gas, for example. The second gasis silane (SiH₄) gas, for example.

Third gas supply unit 233 is configured to be able to supply a third gasincluding ammonia gas. Third gas supply unit 233 is a gas cylinderfilled with the third gas, for example. The third gas is doping gasincluding N (nitrogen atoms). Ammonia gas is thermally decomposed morereadily than nitrogen gas having a triple bond. It is expected that theuse of ammonia gas will improve the in-plane uniformity of the carrierconcentration. Carrier gas supply unit 234 is configured to be able tosupply a carrier gas such as hydrogen. Carrier gas supply unit 234 is agas cylinder filled with hydrogen, for example.

Control unit 245 is configured to be able to control a flow rate of themixed gas supplied from gas supply unit 235 to reaction chamber 201.Specifically, control unit 245 may include a first gas flow rate controlunit 241, a second gas flow rate control unit 242, a third gas flow ratecontrol unit 243, and a carrier gas flow rate control unit 244. Eachcontrol unit may be a MFC (Mass Flow Controller), for example. Controlunit 245 is disposed between gas supply unit 235 and gas inlet port 207.In other words, control unit 245 is disposed in a flow path connectinggas supply unit 235 and gas inlet port 207.

In manufacturing apparatus 200, reaction chamber 201 includes a firstheating region 213 over a region where silicon carbide single-crystalsubstrate 10 is disposed, and a second heating region 214 locatedupstream of first heating region 213. As shown in FIG. 8, second heatingregion 214 is a region from an upstream-side boundary between heatinsulator 205 and heating element 203 to an upstream-side end portion ofthe region where silicon carbide single-crystal substrate 10 isdisposed, in a flow direction of the mixed gas (axial direction ofreaction chamber 201). A boundary portion between second heating region214 and first heating region 213 may be an upstream-side side face of arecess provided in susceptor plate 210. A downstream-side end portion offirst heating region 213 may be a downstream-side boundary between heatinsulator 205 and heating element 203.

In the axial direction of reaction chamber 201, the density of turns ofinduction heating coil 206 may be changed. The density of turns[count/m] is a number of windings of a coil per unit length in an axialdirection of a device. For example, in second heating region 214, thedensity of turns of induction heating coil 206 on the upstream side maybe higher than the density of turns of induction heating coil 206 on thedownstream side, so as to allow effective thermal decomposition ofammonia on the upstream side.

Second heating region 214 may be configured to be able to be heated to atemperature equal to or higher than the decomposition temperature ofammonia. The decomposition temperature of ammonia is 500° C., forexample. The temperature of a portion of heating element 203 that formssecond heating region 214 is 1850 K, for example. In the flow directionof the mixed gas, a length 222 of second heating region 214 is not lessthan 60 mm. In the flow direction of the mixed gas, a length 221 offirst heating region 213 may be greater than length 222 of secondheating region 214.

(Method of Manufacturing Silicon Carbide Epitaxial Substrate)

Next, a method of manufacturing the silicon carbide epitaxial substrateaccording to the present embodiment is described.

First, a step of disposing a silicon carbide single-crystal substrate ina reaction chamber is performed. A silicon carbide single crystal havinga polytype of 4H is manufactured by sublimation, for example. Then, thesilicon carbide single crystal is sliced by a wire saw, for example, toprepare silicon carbide single-crystal substrate 10. This siliconcarbide single crystal has a polytype of 4H—SiC, for example. The 4H—SiChas better electron mobility, dielectric breakdown electric fieldstrength and the like than other polytypes. Silicon carbidesingle-crystal substrate 10 includes an n type impurity such asnitrogen. Silicon carbide single-crystal substrate 10 has n typeconductivity, for example.

The surface of silicon carbide single-crystal substrate 10 is a planetilted at an angle of not more than 4° from a {0001} plane, for example.The tilt direction is a <11-20> direction, for example. Silicon carbidesingle-crystal substrate 10 has a diameter of not less than 150 mm, forexample. Then, silicon carbide single-crystal substrate 10 is disposedin reaction chamber 201. As shown in FIG. 8, silicon carbidesingle-crystal substrate 10 is disposed in the recess in susceptor plate210.

Next, a step of forming a silicon carbide layer on the silicon carbidesingle-crystal substrate is performed. Specifically, manufacturingapparatus 200 described above is used to form silicon carbide layer 20on silicon carbide single-crystal substrate 10 by epitaxial growth. Forexample, after the pressure in reaction chamber 201 is reduced fromatmospheric pressure to about 1×10⁻⁶ Pa, a temperature rise of siliconcarbide single-crystal substrate 10 is started. In the course of thetemperature rise, hydrogen (H₂) gas serving as the carrier gas isintroduced into reaction chamber 201 from carrier gas supply unit 234. Aflow rate of the hydrogen gas is adjusted by carrier gas flow ratecontrol unit 244.

After the temperature of silicon carbide single-crystal substrate 10reaches about 1600° C., for example, a source material gas, a dopant gasand a carrier gas are supplied to reaction chamber 201. Specifically, amixed gas including silane, ammonia, hydrogen and propane is supplied toreaction chamber 201, whereby the respective gases are thermallydecomposed to form silicon carbide layer 20 on silicon carbidesingle-crystal substrate 10.

For example, carrier gas flow rate control unit 244 is used to adjustthe flow rate of the carrier gas (hydrogen) supplied to reaction chamber201 to 100 slm. Second gas flow rate control unit 242 is used to adjustthe flow rate of the silane gas supplied to reaction chamber 201 to 150sccm. Third gas flow rate control unit 243 is used to adjust the flowrate of the ammonia gas to 1.1×10⁻² sccm. In this case, a value obtainedby dividing the flow rate of silane by the flow rate of hydrogen is0.15% in terms of percentage. A growth rate of silicon carbide layer 20is 33 μm/h. In this manner, silicon carbide epitaxial substrate 100including silicon carbide single-crystal substrate 10 and siliconcarbide layer 20 (see FIG. 1) is manufactured.

In order to stretch threading screw dislocation 25 to second mainsurface 12 of silicon carbide layer 20 (epitaxial layer) as shown inFIG. 5, it is desired to adopt the following conditions. Specifically,the thickness of silicon carbide layer 20 is 15 μm, for example. Thegrowth rate of silicon carbide layer 20 is 33 μm/h, for example. Iffirst main surface 11 is 150 mm (6 inches), the in-plane temperatureuniformity (the difference between maximum temperature and minimumtemperature) in the surface of silicon carbide layer 20 is typically 13°C. The in-plane temperature uniformity is desirably not more than 15° C.The C/Si ratio is not less than 1.0 and not more than 1.1, for example.

(Method of Manufacturing Silicon Carbide Semiconductor Device)

Next, a method of manufacturing a silicon carbide semiconductor device300 according to the present embodiment is described.

The method of manufacturing the silicon carbide semiconductor deviceaccording to the present embodiment mainly has an epitaxial substratepreparing step (S10: FIG. 9) and a substrate processing step (S20: FIG.9).

First, the epitaxial substrate preparing step (S10: FIG. 9) isperformed. Specifically, silicon carbide epitaxial substrate 100 isprepared with the method of manufacturing the silicon carbide epitaxialsubstrate described above (see FIG. 1).

Next, the substrate processing step (S20: FIG. 9) is performed.Specifically, the silicon carbide epitaxial substrate is processed tomanufacture a silicon carbide semiconductor device. The “processing”includes various types of processing such as ion implantation, heattreatment, etching, oxide film formation, electrode formation, anddicing. That is, the substrate processing step may include at least oneof the types of processing including ion implantation, heat treatment,etching, oxide film formation, electrode formation, and dicing.

Described below is a method of manufacturing a MOSFET (Metal OxideSemiconductor Field Effect Transistor) as an example of the siliconcarbide semiconductor device. The substrate processing step (S20: FIG.9) includes an ion implantation step (S21: FIG. 9), an oxide filmforming step (S22: FIG. 9), an electrode forming step (S23: FIG. 9), anda dicing step (S24: FIG. 9).

First, the ion implantation step (S21: FIG. 9) is performed. A p typeimpurity such as aluminum (Al) is implanted into second main surface 12on which a mask (not shown) with an opening has been formed.Consequently, a body region 132 having p type conductivity is formed.Then, an n type impurity such as phosphorus (P) is implanted into aprescribed position within body region 132. Consequently, a sourceregion 133 having n type conductivity is formed. Then, a p type impuritysuch as aluminum is implanted into a prescribed position within sourceregion 133. Consequently, a contact region 134 having p typeconductivity is formed (see FIG. 10).

In silicon carbide layer 20, a portion other than body region 132,source region 133 and contact region 134 serves as a drift region 131.Source region 133 is separated from drift region 131 by body region 132.The ion implantation may be performed while silicon carbide epitaxialsubstrate 100 is heated to about not less than 300° C. and not more than600° C. After the ion implantation, silicon carbide epitaxial substrate100 is subjected to activation annealing. The activation annealingactivates the impurities implanted into silicon carbide layer 20, togenerate a carrier in each region. The activation annealing may beperformed in an argon (Ar) atmosphere, for example. The activationannealing may be performed at a temperature of about 1800° C., forexample. The activation annealing may be performed for a period of about30 minutes, for example.

Next, the oxide film forming step (S22: FIG. 9) is performed. Siliconcarbide epitaxial substrate 100 is heated in an atmosphere includingoxygen, for example, to form an oxide film 136 on second main surface 12(see FIG. 11). Oxide film 136 is made of silicon dioxide (SiO₂), forexample. Oxide film 136 functions as a gate insulating film. The thermaloxidation process may be performed at a temperature of about 1300° C.,for example. The thermal oxidation process may be performed for a periodof about 30 minutes, for example.

After oxide film 136 is formed, heat treatment may be further performedin a nitrogen atmosphere. For example, heat treatment may be performedin an atmosphere such as nitrogen monoxide (NO) or nitrous oxide (N₂O)at about 1100° C. for about one hour. Subsequently, heat treatment maybe further performed in an argon atmosphere. For example, heat treatmentmay be performed in an argon atmosphere at about 1100 to 1500° C. forabout one hour.

Next, the electrode forming step (S23: FIG. 9) is performed. A firstelectrode 141 is formed on oxide film 136. First electrode 141 functionsas a gate electrode. First electrode 141 is formed by CVD, for example.First electrode 141 is made of polysilicon including an impurity andhaving conductivity, for example. First electrode 141 is formed at aposition facing source region 133 and body region 132.

Next, an interlayer insulating film 137 is formed to cover firstelectrode 141. Interlayer insulating film 137 is formed by CVD, forexample. Interlayer insulating film 137 is made of silicon dioxide, forexample. Interlayer insulating film 137 is formed in contact with firstelectrode 141 and oxide film 136. Then, oxide film 136 and interlayerinsulating film 137 at a prescribed position are removed by etching.Consequently, source region 133 and contact region 134 are exposed atoxide film 136.

A second electrode 142 is formed at this exposed portion by sputtering,for example. Second electrode 142 functions as a source electrode.Second electrode 142 is made of titanium, aluminum and silicon, forexample. After second electrode 142 is formed, second electrode 142 andsilicon carbide epitaxial substrate 100 are heated at a temperature ofabout 900 to 1100° C., for example. Consequently, second electrode 142and silicon carbide epitaxial substrate 100 are brought into ohmiccontact with each other. Then, a wiring layer 138 is formed in contactwith second electrode 142. Wiring layer 138 is made of a materialincluding aluminum, for example.

Next, a third electrode 143 is formed on third main surface 13. Thirdelectrode 143 functions as a drain electrode. Third electrode 143 ismade of an alloy including nickel and silicon, for example (NiSi, forexample).

Next, the dicing step (S24: FIG. 9) is performed. Silicon carbideepitaxial substrate 100 is diced along dicing lines, for example, todivide silicon carbide epitaxial substrate 100 into a plurality ofsemiconductor chips. In this manner, silicon carbide semiconductordevice 300 is manufactured (see FIG. 12).

Although the method of manufacturing the silicon carbide semiconductordevice according to the present disclosure has been described above withreference to a MOSFET as an example, the manufacturing method accordingto the present disclosure is not limited as such. The manufacturingmethod according to the present disclosure can be applied to varioustypes of silicon carbide semiconductor devices such as an IGBT(Insulated Gate Bipolar Transistor), a SBD (Schottky Barrier Diode), athyristor, a GTO (Gate Turn Off thyristor), and a PiN diode.

(Evaluation)

(Preparation of Samples)

First, a silicon carbide single-crystal ingot is manufactured bysublimation. Specifically, a crucible 33 having a pedestal 31 and anaccommodation portion 32 is prepared (see FIG. 13). Then, a seedsubstrate 34 and a silicon carbide source material 37 are disposed incrucible 33. Silicon carbide source material 37 is disposed inaccommodation portion 32. Silicon carbide source material 37 ispolycrystalline silicon carbide in a solid state, for example. Seedsubstrate 34 is a substrate of a silicon carbide single crystal having apolytype of 4H, for example. The substrate has a diameter of 150 mm, forexample. Seed substrate 34 has a surface 36 and a backside surface 35.Surface 36 is a plane angled off not more than 8° from a {0001} plane,for example. Backside surface 35 of seed substrate 34 is fixed topedestal 31 with an adhesive, for example. Seed substrate 34 is attachedto pedestal 31 such that surface 36 of seed substrate 34 faces a surface38 of silicon carbide source material 37.

Crucible 33 is then heated. Crucible 33 is heated to about not less than2000° C. and not more than 2400° C., for example. Crucible 33 is heatedsuch that the temperature of silicon carbide source material 37 ishigher than the temperature of seed substrate 34 in crucible 33. Then, apressure in crucible 33 is reduced to about not less than 0.5 kPa andnot more than 2 kPa. Consequently, silicon carbide source material 37substantially starts to be sublimated, and starts to be recrystallizedon surface 36 of seed substrate 34. In this manner, a silicon carbidesingle-crystal ingot 40 grows on surface 36 of seed substrate 34 (seeFIG. 14). Silicon carbide single-crystal ingot 40 has a surface 41facing silicon carbide source material 37, and a backside surface 42facing seed substrate 34. Then, silicon carbide single-crystal ingot 40is cut by a wire saw, for example, to provide a plurality of siliconcarbide single-crystal substrates 10.

Next, silicon carbide layer 20 is formed on silicon carbidesingle-crystal substrate 10 by epitaxial growth with the methoddescribed in the above embodiment (see FIG. 8). In this manner, aplurality of silicon carbide epitaxial substrates (see FIG. 1) areprepared. It is noted that the growth of silicon carbide single-crystalingot 40 by sublimation is performed twice. A sample 1 and a sample 2are a plurality of silicon carbide epitaxial substrates 100 obtainedfrom silicon carbide single-crystal ingot 40 after the first growth andthe second growth, respectively.

(Measurement Method)

Next, the numbers of “minute pits” and “carrot defects” in second mainsurface 12 of each of the plurality of silicon carbide epitaxialsubstrates 100 are measured with the aforementioned method.Specifically, WASAVI series “SICA 6X” manufactured by LasertecCorporation is used to measure the numbers of “minute pits” and “carrotdefects” in central region 5 other than outer circumferential region 4within 5 mm from outer edge 3 of second main surface 12. Themagnification of an objective lens is set at ×10. Central region 5 isdivided into a plurality of observed areas. Each of the plurality ofobserved areas is a 1.3 mm×1.3 mm square region.

(Measurement Results)

FIG. 15 is a diagram showing relation between the number of minute pits80 in second main surface 12 and a wafer number of silicon carbideepitaxial substrates 100. As shown in FIG. 15, the density of minutepits is not more than 1000 cm⁻² in silicon carbide epitaxial substrates100 of all wafer numbers. It is shown that silicon carbide epitaxialsubstrates 100 having a wafer number of 10 or less tend to be lower inthe density of minute pits than silicon carbide epitaxial substrates 100having a wafer number greater than 10.

FIG. 16 is a diagram showing relation between the number of carrotdefects 90 in second main surface 12 and a wafer number of siliconcarbide epitaxial substrates 100. As shown in FIG. 16, the density ofcarrot defects is not more than 1.2 cm⁻² in silicon carbide epitaxialsubstrates 100 of all wafer numbers. It is shown that silicon carbideepitaxial substrates 100 having a wafer number of 10 or less tend to belower in the density of carrot defects than silicon carbide epitaxialsubstrates 100 having a wafer number greater than 10. A value obtainedby dividing the number of carrot defects by the number of minute pits isnot more than 1/500 in all of silicon carbide epitaxial substrates 100.

It is noted that each of silicon carbide epitaxial substrates 100 havinga wafer number of 10 or less is a substrate removed from a lower region43 including surface 41 of silicon carbide single-crystal ingot 40 (seeFIG. 14). Lower region 43 is a portion facing surface 38 of siliconcarbide source material 37. A thickness 121 of lower region 43 is aboutfrom 10 mm to 30 mm, for example. By way of example, thickness 121 oflower region 43 is about 10 mm. In other words, lower region 43 is aportion within 10 mm from surface 41 of silicon carbide single-crystalingot 40. Conversely, each of silicon carbide epitaxial substrates 100having a wafer number greater than 10 is a substrate removed from anupper region 44 of silicon carbide single-crystal ingot 40. Upper region44 is a portion facing seed substrate 34.

From the above results, it was confirmed that the number of minute pitsand the number of carrot defects could be reduced in silicon carbideepitaxial substrate 100 using silicon carbide single-crystal substrate10 removed from lower region 43 closer to surface 41 of silicon carbidesingle-crystal ingot 40, as compared to silicon carbide epitaxialsubstrate 100 using silicon carbide single-crystal substrate 10 removedfrom upper region 44 closer to backside surface 42 of silicon carbidesingle-crystal ingot 40.

It should be understood that the embodiments disclosed herein areillustrative and non-restrictive in every respect. The scope of thepresent invention is defined by the terms of the claims, rather than theembodiments described above, and is intended to include anymodifications within the scope and meaning equivalent to the terms ofthe claims.

REFERENCE SIGNS LIST

1 first flat; 2 curvature portion; 3 outer edge; 4 outer circumferentialregion; 5 central region; 6, 7 segment; 8 step-flow growth direction(one direction); 10 silicon carbide single-crystal substrate; 11 firstmain surface; 12 second main surface; 13 third main surface; 14 fourthmain surface (surface); 20 silicon carbide layer; 23 first layer; 24second layer; 25 threading dislocation (threading screw dislocation); 26bottom layer region; 29 surface layer region; 31 pedestal; 32accommodation portion; 33 crucible; 34 seed substrate; 35, 42 backsidesurface; 36, 38, 41 surface; 37 silicon carbide source material; 40silicon carbide single-crystal ingot; 43 lower region; 44 upper region;80 minute pit (groove); 81 first groove portion; 82 second grooveportion; 90 carrot defect; 100 silicon carbide epitaxial substrate; 101first direction; 102 second direction; 111 maximum diameter; 131 driftregion; 132 body region; 133 source region; 134 contact region; 136oxide film; 137 interlayer insulating film; 138 wiring layer; 141 firstelectrode; 142 second electrode; 143 third electrode; 200 manufacturingapparatus; 201 reaction chamber; 203 heating element; 204 quartz tube;205 heat insulator; 206 induction heating coil; 207 gas inlet port; 208gas outlet port; 210 susceptor plate; 211 heating unit; 212 rotationaxis; 213 first heating region; 214 second heating region; 231 first gassupply unit; 232 second gas supply unit; 233 third gas supply unit; 234carrier gas supply unit; 235 gas supply unit; 241 first gas flow ratecontrol unit; 242 second gas flow rate control unit; 243 third gas flowrate control unit; 244 carrier gas flow rate control unit; 245 controlunit; 300 silicon carbide semiconductor device.

1. A silicon carbide epitaxial substrate comprising: a silicon carbidesingle-crystal substrate including a first main surface; and a siliconcarbide layer on the first main surface, the silicon carbide layerincluding a second main surface opposite to a surface thereof in contactwith the silicon carbide single-crystal substrate, an average value ofcarrier concentration in the silicon carbide layer being not less than1×10¹⁵ cm⁻³ and not more than 5×10¹⁶ cm⁻³, in-plane uniformity of thecarrier concentration being not more than 2%, the second main surfacehaving a groove extending in one direction along the second mainsurface, a width of the groove in the one direction being twice or moreas large as a width thereof in a direction perpendicular to the onedirection, and a maximum depth of the groove from the second mainsurface being not more than 10 nm, and a carrot defect, a value obtainedby dividing a number of the carrot defects by a number of the groovesbeing not more than 1/500.
 2. The silicon carbide epitaxial substrateaccording to claim 1, wherein the groove includes a first groove portionand a second groove portion provided continuously with the first grooveportion, the first groove portion is at one end portion of the groove inthe one direction, and the second groove portion extends from the firstgroove portion along the one direction to the other end portion oppositeto the one end portion, and has a depth from the second main surfacewhich is smaller than a maximum depth of the first groove portion. 3.The silicon carbide epitaxial substrate according to claim 1, whereinthe value is not more than 1/1000.
 4. The silicon carbide epitaxialsubstrate according to claim 3, wherein the value is not more than1/5000.
 5. The silicon carbide epitaxial substrate according to claim 1,wherein a density of the carrot defects in the second main surface isnot more than 1 cm⁻².
 6. The silicon carbide epitaxial substrateaccording to claim 5, wherein the density is not more than 0.5 cm⁻². 7.The silicon carbide epitaxial substrate according to claim 6, whereinthe density is not more than 0.1 cm⁻².
 8. A method of manufacturing asilicon carbide semiconductor device, comprising: preparing the siliconcarbide epitaxial substrate according to claim 1; and processing thesilicon carbide epitaxial substrate.